With electric beaters, beat together the ricotta cheese, cream cheese, rum, and sugar substitute, until light and creamy. Set aside.

Combine coffee and Kahlua in a large, shallow dish. Quickly dip one side of 8 of the ladyfinger biscuits into the coffee mixture. Lay closely together, coffee side down, over the base of a large, flat-based serving dish.

Spoon half the cheese mixture evenly over the cookies. Dust with half the cocoa and repeat the layers with the other 8 biscuits, also dipped in the coffee mixture. Finish with a layer of cheese.

Cover with plastic wrap and refrigerate for at least six hours or overnight. Dust with cocoa before serving.

Variations:

Instead of using a large serving dish, use individual parfait glasses. Line the inside rim of the glasses with dipped biscuits and spoon the cheese mixture into the middle. Dust the top with cocoa.